In recent years, active matrix type display devices (those referred to as active matrix type driving image display devices, or simply as display devices) have come into wide general use. In each of those display devices, an active element, such as a thin film transistor or the like, is used as an element for driving pixels disposed in a matrix arrangement. Many kinds of image display devices are able to display high quality images with the use of many pixel circuits and a driving circuit disposed on an insulated substrate. Each of the pixel circuits is composed of active elements, such as thin film transistors (TFT), each formed with a semiconductor film that is actually a silicon film. It is premised here that the present invention uses a thin film transistor typically as each of such active elements.
In case of thin film transistors that use a non-crystal silicon semiconductor film (hereinafter to be referred to as an amorphous silicon film), as has been used generally as a semiconductor film, it has been difficult to compose a circuit that is fast and exhibits enhanced functions. This is because the performance of the thin film transistor, as represented by its carrier (electron or hole) mobility, is limited. And, one of the effective methods that has been employed in order to realize a thin film transistor with fast mobility, as needed to provide an excellent image quality, is to modify (crystallize) a non-crystal silicon film to provide a polycrystalline silicon film (hereinafterto be also referred to as a polysilicon film) in advance, and then to form thin film transistors with the use of this polysilicon film. For this purpose, a laser beam, such as an excimer laser, is irradiated onto the object amorphous silicon film to anneal and modify the film in quality.
FIGS. 19A and 19B show how to crystallize and modify an amorphous silicon film by irradiating an excimer laser beam thereto. The crystallizing method shown in FIGS. 19A and 19B uses a most typical excimer laser beam to scan and crystallize an amorphous silicon film. FIG. 19A shows an example of the configuration of an insulated substrate having a semiconductor layer to be irradiated with a laser beam. FIG. 19B shows how the semiconductor layer is to be modified in quality using the irradiated laser beam. Although the insulated substrate is usually made of a glass plate, it may also be made of a plastic plate.
In FIG. 19B, a linear excimer laser beam ELA, having a width of several nanometers to several hundreds of 100 nm, is irradiated to an amorphous silicon film AS1 deposited on the insulated substrate SUB with an underlayer (made of Sin, SiO2, or the like, not shown) therebetween, and the irradiation position is shifted at intervals of one to a few pulses in one direction (X direction), as shown by the arrow, to scan the film AS1, thereby annealing and modifying the film AS1 that is formed all over the insulated substrate SUB to produce a polysilicon film PSI. The modified polysilicon film PSI is then subjected to etching, wiring, ion implantation, etc. to form each circuit having to active elements, such as thin film transistors, etc. in its pixel region or driving region.
This insulated substrate is used to manufacture an active matrix type image display device, such as a liquid crystal display device, organic EL display device, or the like. If a conventional excimer laser is used to modify a silicon film, many 0.05 to 0.5 μm crystallized silicon grains (polysilicon) come to grow in a random manner at each laser beam irradiated portion. The electron field-effect mobility of a TFT that is formed with such a polysilicon film is about 200 cm2/V·s or under; the average value is about 120 cm2/V·s.
Another method for realizing such a high quality semiconductor thin film is disclosed in the below-listed patent document 1. According to this method, a continuous oscillation laser (CW laser) beam is scanned and irradiated on the object semiconductor thin film in one direction, whereby a long continuous crystal grows in the scanning direction. There is still another method for realizing such a high quality semiconductor thin film. More specifically, a CW laser is irradiated to island-shaped or linearly-shaped semiconductor thin films while the substrate is being scanned. Otherwise, a thermal gradient is generated on the semiconductor thin film in a laser annealing process to obtain a large flat crystal (the crystal is also referred to as an approximate band-shaped crystal semiconductor thin film; hereinafter to be also referred to as a lateral crystal) in which long crystal grains grow in one direction.
The below-listed patent document 2 discloses still another method that uses a continuous oscillation laser. On the other hand, still another method is disclosed in a document other than the patent documents 1 and 2. According to this method, an ELA is irradiated onto the object film through a slit having a width of several micrometers or a mask to generate a thermal gradient thereon so as to produce a lateral crystal. The use of such a semiconductor thin film makes it possible to realize a high performance of more than 300 cm2/V·s in the electron field-effect mobility.
The below-listed non-patent document 1 discloses a method referred to as SELEX (Selectively Enlarging Laser Crystallization). In this method, a continuous oscillation (CW) laser that has been modulated for optimal pulse width and interval is irradiated to each desired region of a semiconductor thin film selectively to generate non-continuous regions of an approximately band-shaped crystal semiconductor thin film having large, high quality crystal grains that are just like continuous grains. If such a high quality semiconductor thin film such as one obtained with the SELAX method is used for a TFT, the electron field-effect mobility of the thin film is improved to provide high performance characteristics, for example, over 350 cm2/V·s. Hereinafter, a flat crystal in which grains grow to be long in one direction will also be described as a lateral crystal.
In the case of crystallization by the laser annealing method as described above, it is required to improve the uniformity of the crystallization property. If the crystallization property is not uniform, it causes imperfect operations of circuits and a lack of uniformity in the screen display. In order to make the crystallization property uniform, therefore, the crystallization state is evaluated in film forming processes as disclosed in the below-listed patent document 3. According to this method, the crystal grains are the maximum in to diameter and granules are not generated yet. At that time, the gloss value of the thin film surface goes down to the minimum. According to the method disclosed in the patent document 3, therefore, in the polysilicon forming process, the reflection rate of the polysilicon is measured to evaluate the diameter of the crystal grains using a non-destructive examination so as to set the optimum crystallization conditions, as well as to reject defective products earlier and set the optimum conditions for the energy density of the laser beam. More specifically, the amorphous silicon film, before the annealing process, has a high reflection rate, since the film surface is smooth. And, if it is annealed into polysilicon, the crystal grains in the polysilicon grow larger in diameter and the surface becomes rough, so that the film reflection rate goes down due to the scattering of the light, etc. In other words, the reflection rate goes down.
The below-listed patent document 4 discloses still another method. According to this method, as shown in FIG. 20, an inspection beam 902 is irradiated to an amorphous silicon film 901 that is treated with an excimer laser beam 900, and transmitted light 903 and a reflected light 904 are detected by detectors 905 and 906, respectively, whereby the progress of crystallization is monitored. The amorphous silicon film 901 is deposited on a substrate 907. Still another method has been proposed in which the Raman scattering spectroscopic method and the X-ray diffraction method are used to evaluate the crystallization property.    [Patent document 1] Official gazette of JP-A No. 86505/2003    [Patent document 2] Official gazette of JP-A No. 86505/2003    [Patent document 3] Official gazette of JP-A No. 274078/1999    [Patent document 4] Official gazette of JP-A No. 144621/1998    [Non-patent document 1] SID Technical Digest 2002, pp. 158-161
In the method disclosed in the above-listed patent document 3, the thin film reflection rate is varied according to a difference among physical properties of the thin films and the smoothness (unevenness value) of the object film surface. Consequently, any method for measuring the reflection rate in such a way cannot apply to the evaluation of the film surface smoothness, grain diameters, and various unevenness values (defective density, oxygen content, hydrogen content concentration, film thickness, etc.), that is, physical properties of the object thin film, as well as a difference between laser annealing conditions and crystallization results determined by those items. And, the reflection measuring method is just effective for average evaluations; the accuracy is not sufficient. In addition, in such reflection rate measurements, it is not easy to define the measuring angle, etc. accurately. The method is thus inconvenient for operations.
Furthermore, the method disclosed in the above-listed patent document 4 cannot detect slight changes in the state of once-crystallized polysilicon, while it can detect significant state changes from amorphous silicon to polysilicon. And, what should be watched here is that the methods disclosed in the above-listed patent documents 3 and 4 cannot distinguish between lateral crystal and granular crystal, so that these methods are unsuitable for evaluating the crystallization property of lateral crystals.
The evaluation by either the Raman scattering spectroscopic method or the X-ray diffraction method requires a large and complicated measuring apparatus, to the extent that it would be difficult to install the apparatus in a small space. The measurement according to these methods also requires much time, so that evaluations in real time are not available. Those are the problems of the conventional techniques which are required to be solved.